1. Field of the Invention
This invention relates generally to a semiconductor bulk acoustic resonator (SBAR) and, more particularly, to a thin film voltage-tuned semiconductor bulk acoustic resonator.
2. Discussion of the Related Art
In many electrical applications, it is desirable to utilize high frequency (i.e., greater than 1 GHz) voltage-controlled oscillators (VCOs). As is well understood, high frequency voltage-controlled oscillators typically have a poorer phase noise ratio than single-frequency oscillators which are stabilized by acoustic resonators. This is because the voltage-controlled oscillators contain high 1/f noise components, such as varactors, which help determine the output frequency. To eliminate the noise problems inherent in varactors, the varactors can be combined with high-Q (quality factor) acoustic resonators to create narrowband VCOs with relatively low phase noise. However, the tuning range of these VCOs are typically only a few hundred parts per million (ppm) and the frequency control linearity is not good.
In order to eliminate this phase noise problem and the narrow tuning range, it is possible to vary the frequency of crystal-controlled oscillators by applying and varying a voltage potential between the oscillator's electrodes. This results in creating a VCO that has phase noise ratios as low as an equivalent single-frequency oscillator with highly linear control characteristics. Unfortunately, this type of VCO is not practical because an extremely high voltage is required to vary the frequency. For example, a quartz crystal resonator having a thickness of one millimeter requires 1,000 volts to produce an electric field of one million volts per meter (V/m). As such, this technique is not practical, nor utilized for crystal-controlled oscillators since an unreasonably high voltage is required to make any useful frequency change.
Each of the above-mentioned techniques produces voltage-controlled oscillators having low phase noise and relatively linear characteristics. However, each of these techniques have several drawbacks associated with their use that effect their practicality, cost, size, complexity and power consumption. What is needed then is a voltage-tuned acoustic resonator that has low phase noise, highly linear characteristics and can be varied over a broad frequency range by utilizing practical DC bias voltages. This will, in turn, reduce the cost, size, complexity and power consumption of voltage-controlled oscillators as well as tunable filters. It is, therefore, an object of the present invention to provide such a device.